In recent years, in compression refrigerating machines such as an air conditioner, a car air conditioner, etc., a saturated fluorinated hydrocarbon (HFC) having 1 or 2 carbon atoms, that is a refrigerant which does not destruct an ozone layer, is used. However, recently, these compression refrigerating machines have been frequently used under a long-term severe condition more and more.
In consequence, a lubricating oil composition which is used for such a compression refrigerating machine is demanded to have excellent heat/oxidation stability.
In addition, conventionally, mixed refrigerants containing difluoromethane (R32), for example, R410A, R407C, etc., have been used as an HFC refrigerant which does not destruct an ozone layer, for various compression refrigerating machines inclusive of an air conditioner.
However, furthermore, in view of the fact that in addition to protection of an ozone layer, it was considered to be necessary to prevent the global warming, the use of a refrigerant containing a higher amount of R32 having a low global warming potential has been desired.
Since R32 is relatively high in a theoretical COP or a heat transfer coefficient and low in a pressure loss of a refrigerant, it has such properties that when used for air conditioning apparatuses, it is high in energy efficiency.
However, R32 has such properties that the discharge temperature of a compressor is higher by substantially about 20° C. than that of conventional R410A and R407C.
Under those circumstances, studies for improving structures of refrigerating apparatus are conducted (see, for example, PLT 1).
In addition, for lubricating oils for compression refrigerating machine using such a refrigerant, a lubricating oil composition having high heat/oxidation stability in the presence of such a refrigerant is demanded.
However, different from unsaturated HFC or iodides, saturated HFC has high stability and was considered to be able to respond to such a demand by adding an existing antioxidant or acid scavenger which is used in R134a and the like (see, for example, PLTs 2 to 4). However, even when an existing antioxidant or acid scavenger is merely added, or its adding amount is increased, an effect for increasing the heat/oxidation stability could not be satisfied.